DONOVAN MCNABB BEGINS JAIL SENTENCE … Behind Bars In Arizona

Donovan McNabb — is now Inmate #T231133 … ’cause the ex-NFL star has officially begun his jail sentence in Arizona stemming from a DUI arrest back in June, TMZ Sports has learned.

We’ve confirmed McNabb has turned himself in to begin his 18 day sentence … sporting a fresh Nike jacket.

But as we previously reported, McNabb will NOT spend his days in jail — the judge granted him “work release” which means he can leave the facility to work during the day, but must return at night to sleep in a cell.

McNabb was arrested on June 28th after crashing into a car that was stopped at a red light … cops say his blood alcohol was more than TWICE the legal limit.